Alpha image or model?
For a close-up I would certainly model and texture the leaves, however, at a distance a simple alpha texture could get just as realistic of a result with much less computing power. For example if you are taking a closeup of some foliage and then zooming/focusing on a subject, modelling the extra detail would be worth the extra effort/render time. However if you just want some realistic shrubs/trees in the background, textures are the way to go. And if it is only for a very distant scene or the background is blurred, having textures for entire sections of foliage might help even more. 

Basically, unless you can see the difference, there is no need for the extra polys in your scene.